If you downloaded from wordpress.org, it is almost certainly a false positive. If you download from the release archive, there are md5 and sha1 hash values with which you can compare against a hash of the downloaded file. If the values match, you are assured the file remained untainted in the download process.
Depending on how much detail is given about where the offending code is, you can also compare your file against the version at core.trac.wordpress.org/browser/tags/3.8.1/src to confirm nothing unusual has worked its way into your version.